A case study of the effect of grain size on the oxygen permeation flux of BSCF disk-shaped membrane fabricated by thermoplastic processing

Abstract

Oxygen permeability measurements of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) disk shaped membranes fabricated by thermoplastic processing and sintered at different temperatures (1000–1100 °C), showed no influence of the grain size on the oxygen permeation fluxes. To further investigations, Electron Backscattered Diffraction (EBSD) and Conductive mode (CM) microscopy methods were used for texture analysis and observation of the local electrical behavior in the BSCF membranes, respectively. EBSD results revealed that the grain size of the membranes increased with increasing the sintering temperature from an average of 3.32 μm at 1000 °C to 18.25 μm at 1100 °C. Also, it was seen that there was no textural difference between the different samples. CM analysis demonstrated that the electronic conductivity of the grains and grain boundaries was similar in the membrane sintered at 1000 °C. Finally, the stability of the membrane under the operation conditions was tested, and it was found that the permeation flux was nearly constant at 900 °C after an operation time of more than 50 h, whereas oxygen permeation flux declined after a relative short time at 825 °C.

Introduction

Perovskite-based, mixed ionic–electronic conductors (MIEC) are of great interest because of their 100% oxygen permselectivity at elevated temperatures. These materials are widely under development as ceramic membranes for oxygen separation and in catalytic partial oxidation reactors, as electrodes for solid oxide fuel cells and for high temperature electrolysis [1], [2], [3], [4]. Oxygen permeation in MIEC membranes is driven solely by a difference of oxygen partial pressure (oxygen potential gradient) on either side of the membranes.

Since Teraoka reported high oxygen permeation flux in La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF) ceramic membranes in the late 1980s [5], [6], a large number of studies have been carried out to optimize oxygen permeation in this system, with SrCo_0.8Fe_0.2O_3−δ (SCF) showing significant promise. However, later studies revealed that the SCF phase was not stable below 800 °C at oxygen partial pressures lower than about 0.1 atm due to oxygen vacancy ordering [5], [7], [8], [9], [10].

It was later found that the structural and chemical stability of the SCF membranes were greatly improved by partial substitution of Sr with Ba, retaining a high oxygen permeation flux for compositions around Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF). Recently, BSCF has received increasing attention for use in ceramic oxygen membranes [11], [12], [13], [14], [15].

It has been reported that the oxygen permeation flux of the MIEC membranes is sensitive to microstructural features, such as grain size and grain boundary structure [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The microstructure depends both on processing history, and especially the sintering procedure, even for similar components. The preparation of MIEC membranes can be considered in terms of three steps: (1) powder synthesis, (2) shaping, and (3) sintering. The influence of each of these steps on the microstructure and oxygen permeation flux of the MIEC membranes has been investigated over the last decade, with several different behaviors reported [13], [16], [17], [19], [20], [21], [24].

In the case of BSCF, previous studies have resulted in conflicting data, especially with regard to the influence of grain size on permeation [13], [14], [18], [22], [23], [24], [25], [26]. The effect of the microstructural on the oxygen permeation flux cannot be generalized and the contradictions could be attributed to microstructural variations caused by differences in powder synthesis routes and sintering conditions used by different groups.

In this study, we have examined the effect of grain size on the oxygen permeation flux of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes, fabricated by warm pressing. Three samples with differing grain sizes were prepared by sintering at different temperatures. The associated microstructural changes were characterized using scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) for texture analysis and conductive mode (CM) microscopy to observe the local electrical behavior in the BSCF membrane. Oxygen permeation experiments were also carried out on the membranes sintered at each temperature.